Thermotherapy such as hyperthermia, thermal ablation, and thermal cancer necrosis is one of cancer treatment modalities. Such thermotherapy treats cancer through the method of destroying tissue in area to be treated by heating the area to be treated above a certain temperature.
For example, a specific method of thermotherapy includes a technique called “dielectric heating”, which heats the area to be treated by applying a high-frequency current thereto, and focused ultrasound technique, which heats the area to be treated by focusing ultrasound thereon.
In performing such a thermotherapy, there may be a rise in temperature even in the non-target area around the target area to be treated at the same time, which may influence the therapeutic effect, thus it is required to measure the temperatures of a wide range of tissues including the target area to be treated as well as the non-target area rapidly and accurately.
Conventionally a temperature measurement in the thermotherapy has been performed by positioning a probe such as a thermocouple at and around the area to be treated. However, for such an invasive temperature measurement technique, there might be a concern about impact on the non-target area, thus there has been a need for a noninvasive temperature measurement technique.
Accordingly, as disclosed in a patent document 1, a method for measuring and imaging the tissue temperature distribution by means of water-proton nuclear magnetic resonance signals (water signals) has been proposed.
Such a method for imaging the temperature distribution focuses on a water proton chemical shift in phase mapping or nuclear magnetic resonance spectroscopic imaging by means of NMR equipment to image variation in temperature thereof. This method is now practically used only in temperature distribution measurement of for example less fluctuating high-water content tissue.
In contrast, for example in focused ultrasound therapy of breast cancer, the target tissues to be measured is the mammary gland, which is high-water content tissue, and the breast, which is a mixture of high-water content tissue and fat tissue.
Imaging of the temperature distribution in fat tissue with low water content by acquiring a water signal, however, has not been practically used in view of incomplete signal separation, partial volume effect, and signal-to-noise ratio (hereinafter, simply referred to as S/N ratio).
Therefore, as described in non-patent documents 1 to 3, a technique for measuring the temperature of fat tissue by means of a NMR signal has been proposed.
This measuring method enables the temperature measurement using the longitudinal relaxation time (T1) and intensity of an integrated signal over fat tissue without observing the frequency spectrum (chemical shift) of fat tissue.